Field
Embodiments of the disclosure relate to gas component separations using polymer membranes. In particular, embodiments of the disclosure relate to compositions of and methods for using block co-polyimide membranes with high gas permeability and selectivity for separating components of natural gas.
Description of the Related Art
Worldwide demand for natural gas has increased over the past several decades, as it is a cleaner and more efficient fuel than certain other hydrocarbons, such as coal. This trend is expected to continue as traditional liquid fossil fuel reserves become depleted and concerns about carbon dioxide (CO2) emissions from coal combustion grow. Since raw natural gas from various origins is different in composition, the potential growth in the demand for methane (CH4) requires separation technologies with increased efficiency. Various impurities exist in raw natural gas, and sub-quality “sour gas” containing high levels of these components must be upgraded to meet pipeline specifications and environmental limitations.
Although methane constitutes the key component of natural gas, it may also contain considerable amounts of impurities including water, carbon dioxide (CO2), nitrogen (N2), hydrogen sulfide (H2S), and other hydrocarbons. In current systems, natural gas treatment involves the removal of acid gases, such as CO2 and H2S, before natural gas is delivered to pipelines. At high concentrations, acid gases can corrode transportation pipelines and create numerous other detrimental effects. Moreover, H2S is extremely toxic, and its combustion produces harmful sulfur dioxide (SO2) gas. Sweetening of natural gas, therefore, is necessary to reduce pipeline corrosion, prevent atmospheric pollution, increase the fuel heating value of the gas, and decrease the volume of gas to be transported.
Current natural gas treatment includes many industrial gas separation processes. Absorption of acid gases in basic solvents, such as liquid amines and hot aqueous potassium carbonate solutions, and pressure swing adsorption (PSA), are examples of natural gas purification technologies. A number of drawbacks to these processes exist, as they rely upon energy-intensive thermal regeneration steps, large environmental footprints, heavy maintenance requirements, and high capital costs. As such, membrane-based separations and hybrid absorption-membrane processes have received much attention recently due to advantages in energy efficiency, process footprint, operational flexibility, and reduced environmental impact. Limited data have been reported on the development of membrane materials for aggressive sour gas separations, such as, studies on H2S/CH4 separation performance using rubbery membranes. Adequate performance has been found in some studies. However, since rubbery materials separate based on solubility selectivity, the CO2/CH4 separation efficiency of these rubbery polymers tends to fall significantly below glassy polymers such as cellulose acetate (CA) and polyimides, which separate molecules primarily based on size.
A variety of processes and techniques have been developed to separate and recover helium from multicomponent gas streams. Such processes include stand-alone membrane units, stand-alone cryogenic units, and combinations of membrane units and PSA units. Stand-alone cryogenic processes have been used to produce crude helium at high recovery rates from natural gas and other streams containing low purity helium. When the concentration of helium in the feed drops to low levels, for example below about 1 mol % concentration, processes using stand-alone cryogenic units become impractical. Helium is typically present in natural gas at below 0.5 mol % concentration levels and is mostly extracted as crude helium across liquid natural gas (LNG) trains. World demand for helium is increasing, and this is expected to put pressure on production facilities as demand for high-purity helium products begins to outstrip supply. In light of these trends, processing methods that overcome the impracticality of the classical processes described above are needed. Separation of helium from natural gas using high-performance membranes or a combination of membranes with any of the other classical processes described can improve the economics of helium recovery.
Glassy polyimides constitute a large portion of recent high-performance membrane materials for acid gas separations and helium recovery from natural gas. These materials exhibit high-glass transition temperatures (Tg) (Tg greater than about 200° C.) and are relatively hydrophobic. The majority of glassy polyimide acid gas and helium removal capacity is derived from size selectivity. For acid gas separations, these materials frequently give superior efficiency, productivity, and resistance to penetrant-induced plasticization compared to cellulose acetate (CA), which is presently the industrial standard membrane material for CO2 separations.